The invention is situated in the field of the packaging technology. It concerns a method and a device that serve for coating a polymer film with an oxide layer, in particular with a barrier layer of silicon oxide (SiOx).
Polymer film or sheet material e.g. of PET (polyethylene terephthalate), PA (polyamide), PP (polypropylene) or PE (polyethylene) is coated with a thin barrier layer of silicon oxide for improving the barrier properties thereof, i.e. for reducing its gas permeability, in particular its permeability for oxygen, water vapor, carbon dioxide, and aroma compounds. Polymer films usually have a thickness of e.g. 1 to 100 μm, polymer sheets are thicker. In the following, the term polymer film is used to include not only the very thin material but also the thicker material, which is usually termed sheet material.
The barrier layer has a composition of SiOx and may also contain hydrogen, carbon and/or nitrogen. The coated polymer film is useful for a number of packaging applications, e.g. for cardboard brick packages, for beverages in which the polymer film forms an inner lining. The transparency of the barrier layer allows for its use in transparent packaging materials also, e.g. in sheet materials for pouches or for tray lids.
According to the state of the art, SiOx barrier layers, as shortly described above, are deposited on polymer film e.g. in a PECVD-process (plasma enhanced chemical vapor deposition process) wherein a web of the polymer film, usually supported by a rotating drum, is transported through a plasma being sustained at a reduced pressure and wherein a process gas mixture containing an organosilicon compound (e.g. hexamethyldisiloxane or HMDSO) or an inorganic compound containing silicon (e.g. SiCl4, SiH4) is fed into the plasma. Reactive silicon based particles which are produced in the plasma by decomposition of the organosilicon compound are deposited on the exposed surface of the polymer film to form the barrier layer. In particular for coating a web of the polymer film being supplied in rolls, the reduced pressure of the process necessitates equipment of high cost and the process needs to be interrupted for each roll exchange (non-continuous or batch roll-to-roll process).
Plasma enhanced deposition processes carried out at ambient pressure, for producing barrier layers (DBD-process or dielectric barrier discharge process), also belong to the state of the art. Obviously, a device for carrying out such an ambient pressure process is considerably simpler than the device needed for the reduced pressure process, and rolls can be exchanged without interruption of the process (continuous roll-to-roll process). However, long development work has not been able to reduce energy and process gas consumption of the ambient pressure plasma process to an acceptable level and the necessary high power density makes it very difficult if not impossible to produce large quantities of coated polymer film having a constant high quality.
Other known processes for producing silicon or aluminum oxide barrier layers are evaporation and reactive evaporation which are also carried out at reduced pressure and therefore have the same disadvantages as the above mentioned reduced pressure PECVD-process.
It is further known to treat substrate surfaces by exposing the surface to a flame. Such treatment is applied for giving the substrate a hydrophilic or adhesion promoting surface as is used for printing, lacquering or gluing surfaces of polymer, glass or metal substrates.